As a geologist and conservation scientist, I am often asked about
the remarkable travertine stone exterior of the Getty Center. What
kind of stone is travertine? Why are there fossils in it? How will
it look in 20 years?

Travertine is a product of the earth's water and carbon cycles.
As carbon dioxide-rich rainwater percolates through soil and stone,
it slowly dissolves tremendous quantities of limestone along underground
fissures. Reemerging at the surface as a spring (now saturated with
dissolved limestone), this water releases carbon dioxide gas into
the atmosphere—much like carbonated mineral water. Because of
this "Perrier effect," the limestone can no longer remain in solution.
It recrystallizes, typically as the water cascades over organic
films made of bacteria, algae, and mosses. A dense, banded carbonate
stone is built up over time as new material covers older layers.

Calcite and gypsum, the minerals that make up about 99 percent
of travertine stone, are colorless. The beautiful honey color of
the Getty Center travertine actually has its origin in the other
1 percent of the stone: traces of yellow sulfur, brown iron compounds,
and organic pigments. The intricate "Swiss cheese" texture of travertine
is partly the result of gas bubbles, which are often trapped between
layers of stone, creating spherical voids. Minerals crystallizing
on the ever-present bacteria in travertine deposits—like granular
snow blanketing a miniature landscape—preserve organic growth forms,
called "shrubs," and produce much of the rugged relief evident across
the stone's surface. In some cases, travertine layers are similar
to tree rings, with lighter and darker laminations representing
seasons of growth.

A view of the Italian quarry that
is the source of the travertine stone used at the Getty
Center. Photo: Jim Mawson.

Travertine is found in greatest abundance where hot and cold springs
have been active for tens of thousands of years. The most famous
travertine location, and the source of the stone used for the Getty
Center, is Bagni di Tivoli, 20 kilometers east of Rome, where travertine
deposits over 90 meters thick have been quarried for over two thousand
years.

Because travertine is plentiful, weighs less than marble or granite,
and is relatively easy to quarry, it was the stone most commonly
used by the ancient Romans. Famous structures constructed with Tivoli
travertine include the Colosseum, the Trevi Fountain, the colonnade
of Saint Peter's Basilica, and many Roman aqueducts. In our century,
Lincoln Center in New York and the ABC Entertainment Center in Los
Angeles are faced with travertine from the same Tivoli quarries.

While the age of the stone used for the Getty Center is unknown,
it probably dates from about 8,000 to 80,000 years ago. The Center's
travertine is split with the grain of the stone, making visible
many more fossils than are seen in the more common banded travertine,
which is cross-sectioned and polished. At least two species of fossilized
leaves are fairly common at the Center—evidence of a lakeside environment
at the time of the stone's formation. The impression of a feather
is preserved in stone at the foot of the curved East Building wall;
an unusual bone embedded in a travertine block has also been discovered.
The rapid deposition of the travertine layers acts as a natural
preservative for these traces of evanescent prehistoric life. Paleontologists
at the George C. Page Museum in Los Angeles are working with Getty
scientists to identify these fossils.

Fossilized imprints of a
feather and leaves in two of the Center's travertine stone
blocks. Photos: Dusan Stulik.

An electron micrograph
of calcite crystals in a block of travertine stone.
Photo: Eric Doehne.

Over 108,000 square meters of Roman Classic travertine from the
Lippiello family quarry at Bagni di Tivoli were used at the Getty
Center. In order to remove the travertine from the vertical quarry
face, workers drill holes into the stone, outlining a block 6 meters
high, 12 meters wide, and 2 meters deep. A diamond-studded cable
is then threaded through the holes, lubricated, and pulled against
the stone with a set of pulleys. A large cut may take a day and
a half, but eventually diamond wins out over the softer travertine.
When the cuts are completed, the slab is pushed away from the quarry
wall and falls onto mounds of earth, which help cushion the fall.
The slab is then broken up into more manageable cubes, which are
taken to the Carlo Mariotti factory for honing and splitting. An
automated guillotine was created by Mariotti to split the stone
along its natural bedding plane. On the average, each block at the
Getty Center is 76 x 76 centimeters and weighs 115 kilograms, with
a typical thickness of 8 centimeters. About three hundred thousand
pieces of stone were used for facades and paving.

The travertine floor surfaces are anticipated to last at least
50 years before replacement in high-traffic areas is needed. High-quality
travertine is very durable, since it is formed at the earth's surface
in relative equilibrium with the environment. Most other building
stones are formed under different conditions, deep underground.

Accelerated aging tests of the travertine stone were undertaken
by two consulting firms and the GCI to determine the suitability
of the stone, the mounting system, and the chemical treatments.
Each stone block at the Getty Center has been treated with a silicon-based
water repellent that is expected to ease cleaning. Paving stone
and walls in public areas up to a height of 2.1 meters have also
been treated with an oil-resistant coating to reduce soiling. The
frequency and methods used to remove dust and soot from the cleft-cut
stone will depend on soiling and weathering rates that have yet
to be studied fully. Over time, the honey color of the fresh travertine
on the Getty Center will change as the stone weathers and a natural
patina forms.

In short, the beautifully colored and textured travertine at the
Getty Center will offer visitors a wonderful opportunity to appreciate
the structure, genesis, and natural history of the stone.

Eric Doehne is an Associate Scientist with the GCI Scientific
Program.